The battery pack does a bunch of stuff: - adds longer shooting time with a second battery - gives a second set of buttons in portrait orientation - increases the frame rate with the right batteries - makes the camera bigger - adds weight
The key part for me is the second set of buttons for portrait orientation. This makes life so much nicer. I did not have a grip for my D200 and longed for the extra buttons.
The extra battery life is nice if you are shooting all day events. If you want to save on weight, you can leave out the battery and the buttons on the grip still work.
I have big hands and like the bigger size when it is attached.
Renting one is a great way to see if the grip is something you will like or not.
---------+---------+---------+---------+ Joseph K Seattle, WA, USA